High speed steel



United States Patent Ofilice 3 ,113,862 Patented Dec. 10, 1963 3,113,862 HIGH SPEED STEEL Richard F. Harvey, Pittsburgh, and Charles W. Schuck, New Kensington, ya, assignors to Eraeburn Alloy Steel Division Continental Copper and Steel Industries, a corporation of Delaware No Drawing. Filed May 11, 1960, Ser. No. 23,193 5 Claims. ((11. 75-126) This invention relates to ferrous alloys, particularly high speed steels and more particularly to compositions which are especially suitable for cutting tools. This invention pertains to a forgeable molybdenum-tungstenvanadium-cobalt-iron base alloy which is characterized by high hardness, excellent red hardness, and outstanding cutting properties.

At the present time there are a number of hi h speed steels of varying composition which are used for cutting tools such as milling cutters, reamers, tool bits, drills, end mills, planing tools, saws, knives, etc. Conventional high speed steel compositions as a rule may be heat treated to a maximum hardness of about 67 to 68 Rockwell C. High hardnesses up to about 68 Rockwell C are obtained with relatively high carbon contents of about 1.5% and above. At such high carbon levels the steels possess a relatively low degree of toughness.

The conventional high speed steels also posses inadequate red hardness for such requirements as cutting high temperature alloys and at the higher cutting speeds they will not stand up and will burn out. Ceramics or sintered carbide tools are often required in this range of higher cutting speeds beyond that at which conventional high speed steels may be used. Ceramics or sintered carbides are also required on many of the difiicult to machine complex alloys.

In the present invention higher hardnesses above about 68 Rockwell C and up to about 70 Rockwell C are readily attainable. The cutting performance of tools made according to the teachings of this invention are outstanding and it has been found that the use of such tools extends the range of cutting speeds beyond that heretofore found useful with conventional high speed tools.

It should be emphasized that a higher tempering temperature is required to temper steel of the present invention to a given hardness level and with the higher tempering temperature a greater degree of toughness results. It is recognized that maximum hardness is often not the best hardness for a given cutting application and it is frequently desirable to temper to lower hardness levels.

We have discovered a ferrous alloy which provides a high speed composition capable of being heat treated to very high hardnesses in the range of about 68 to 70 Rockwell C and characterized by a high degree of toughness at these high hardness levels.

We have also found that the alloy of this invention is characterized by a markedly higher degree of red hardness of resistance to softening at elevated temperatures than was available heretofore as well as by excellent cutting properties, particularly on cutting highly alloyed materials at high hardness levels and at high cutting speeds.

The alloy of this invention is thus able to extend the useful range of high speed steels and to provide useful cutting tools capable of cutting under conditions hereto fore thought practical only with ceramic tools or carbides.

High carbon-high vanadium high speed steels such as T-lS with about 1.6% carbon and 5% vanadium have been made before and such steels may be heat treated to hardnesses up to about 67 and occasionally 68 Rockwell C. T-lS and its molybdenum base counterpart M-15 are recognized as representing the highest hardness obtainable on commercially available high speed steels.

What has now been discovered, and this is the basis of the present invention, is that very high hardnesses in the range of about 68 to 70 Rockwell C may be readily obtained with carbon contents less than about 1.5% along with increased resistance to tempering at elevated temperatures and very superior cutting characteristics as compared with the teachings of the prior art. These desirable results are obtained on high speed steels containing less than about 1.5% carbon and preferably less than 1.3% carbon as long as the molybdenum, tungsten, vana dium and cobalt are in definite proportions in accordance with the teachings of this invention as will be more fully described hereinafter.

The capability of heat treating to high hardness levels and the excellent cutting properties are obtained with the carbon content preferably less than 1.3% and with the vanadium content about twice the carbon content in percentage by weight. The molybdenum should be in excess of tungsten and the ratio of the molybdenum to tungsten should be in the ratio of about 6.25 to 5.25. Also the cobalt should be preferably about 12. The chromium should be about 4.25% and the manganese and silicon should be low, preferably less than about 1.00% each.

It shall be noted that contrary to conventional practice where vanadium to carbon ratios of about 3 to 1 or higher are used, in the present invention a much lower ratio has been found to be beneficial. This is an important consideration from the cost standpoint as vanadium is an expensive alloying element which presently costs about $3.30 per pound of contained vanadium in vanadium ferroalloys.

While it is not desired to be limited by the consequences of a theory we believe that precipitation hardening occurs as well as martensitic hardening which probably explains the exceptionally high hardness obtained. In this connection it has been noted that alloys of the present invention do not immediately develop maximum hardness but this condition of maximum hardness occurs after aging for several weeks.

To investigate the aging factor further a series of experimental heats were melted as tabulated in Table 1. In these alloys at low carbon content was purposely used to avoid martensitic hardening and to investigate separately the precipitation hardening effect.

In order to compare the precipitation hardening effect of the alloys listed in Table 1 the Rockwell hardness was compared after a standard treatment consisting of solution treating at 2200 F. followed by aging or tempering at 1250 F. It has been found also that the ratio of cobalt to equivalent tungsten content has a profound effect on precipitation hardening and this ratio was also tabulated for the alloys in Table 1.

In determining the equivalent tungsten content since molybdenum is twice as potent as tungsten the following formula was used:

denum as compared with tungsten is in the ratio of their respective atomic weights.

3, TABLE I Low Carbon Heats Melted to Investigate Precipitation Hardening Efiect Precipitation Hardening Efiect Ratio of Roewell Heat Cobalt to Equivalent Hardness 1 Tungsten Heat A 602 55 Heat B .745 45 Heat C-.. 1.12 43 Heat D 1.460 33 1 Solution treated, aged 1200 F.

The precipitation hardening eifect of Heats A, B, C and D is compared in the chart FIGURE #1. It wil be noted that the precipitation hardening effect is very powerful when the ratio of cobalt to equivalent tungsten is less than .75. Also this ratio should be preferably over 0.5.

Accordingly the composition of the present invention has been balanced so that the ratio of cobalt to equivalent tungsten is about .68 at which ratio precipitation hardening occurs in addition to martensitic hardening. It has also been discovered that the preciptation hardening occurs substantially uniformly throughout the entire section of the present composition. This is very important and beneficial to the physical properties as very many precipitation hardening alloys have low physical properties because the precipitation is concentrated at the grain boundaries resulting in weakness and brittleness.

The sulphur and phosphorus are conventionally held to less than about .05% except where sulphur is intentionally added for free machining qualities in which case it is usually held to about .06/.20%.

Considering only the essential elements the preferred analysis which is capable of being heat treated to high hardnesses over about 68 Rockwell C and which possess excellent cutting qualities is:

Percent by weight Carbon 1.10 Chromium 4.25

Vanadium 2.25 Molybdenum 6.25 Tungsten 5 .25 Cobalt 12.00

The balance is substantially iron with the manganese, silicon, sulphur and phosphorus together with residual impurities in conventional amounts.

The composition of the present invention may, however, extend over a relatively wide range and retain its desirable high hardness and good cutting qualities. Broadly the composition may fall within the limits of the following range of concentrations of alloying elements.

Percent by weight Carbon About .901.5. Silicon About 1% maximum. Manganese About 1% maximum. Sulphur About .40% maximum. Phosphorus About .20% maximum. Chromium About 3.25 to 6.25. Tungsten About 3.25 to 7.25. Molybdenum About 4.25 to 8.25. Vanadium About 1.25 to 3.25.

Cobalt About 9 to 14.

While the alloy of this invention may fall within the range of concentrations of alloying elements enumerated above and still retain its desirable characteristics of high hardness and good cutting qualities it is desirable to limit the composition to a narrower range of elements for more economical and effective use of the alloying elements. Therefore a narrower, preferable range of composition limits is as follows:

Carbon About 1.00 to 1.30. Silicon About .60% maximum. Manganese About .60% maximum. Sulphur About 30% maximum. Phosphorus About .10% maximum. Chromium About 3.25 to 5.25. Tungsten About 4.25 to 6.25. Molybdenum About 5.25 to 7.25. Cobalt About 11 to 13. Vanadium About 1.75 to 2.75.

For reference a typical preferred melting range for high speed steel of the present invention is:

Percent Carbon 1.05 to 1.15 Silicon .15 to .35 Manganese .15 to .35 Sulphur .15 maximum Phosphorus .03 maximum Chromium 4 to 4.5

Tungsten 5 to 5.5 Molybdenum 6 to 6.5 Vanadium 2 to 2.5

Cobalt 11.5 to 12.5

A heat of steel made according to our invention was tested to show that the desirable properties including high hardness, high red hardness, and excellent cutting characteristics were enhanced.

High speed steel of the composition represented by Heat No. 1 responded very well to hardening and tempering resulting in very high hardnesses as tabulated herewith. For reference comparable values are listed for a 1.60% carbon T-15 and an M-2 standard type of high speed steel.

Present Rockwell C Hardness Invention, Heat N o. 1 Hardened, T-15 M-2 2,210 F. Hardened, Hardened,

Tempered 1,025 F 70 67 65 Tempered 1,075 F 68% 66% 64 Tempered 1,125 F 66% 65 62 The hardness values for high speed steel made in accordance with the present invention will be recognized by those skilled in the art as being exceptionally high and an improvement over the prior art. Also the new composition can be readily forged, annealed, machined, heat treated and ground.

EXAMPLE 2 A heat of steel was made to the following composition:

Percent Carbon 1.54 Silicon .39 Chromium 4.29 Vanadium 2.17 Tungsten 3.71 Molybdenum 4.65 Cobalt 1 1.60

R.p.m. 90 Depth of cut inches .015 Feed -3 do .025

Heat No. 1 out an average of 146 surface feet per minute at which speed tool failure occurred.

By comparison with the above results a standard T- steel hardened at 2300" F. and tempered at 1050 F. for two hours plus 1020 :F. for two hours to 'a Rockwell C hardness of 66 cut 116 surface feet per minute at which speed tool failure occurred.

EXAMPLE 3 A heat of steel was made to the following composition:

Percent Carbon 1.14 Silicon .24 Tungsten 4.03 Molybdenum 5.05 Chromium 4.37 Vanadium 3.02 Cobalt 12.48

Test tool bits wene hardened from 2250 F. followed by tempering at 1050 F. for two hours plus 1020 F. for two hours to a Rockwell hardness of C 69.

These test bits cut an average of .133 surface feet per minute at which speed tool failure occurred.

Cutting tests conducted on tool bits of high speed steel made in accordance with the principles of this invention have been outstanding on conditions of interrupted cuts as well as on continuous cutting. The cutting qualities of high speed steels made in accordance with the teachings of this invention are particularly good on cutting highly alloyed materials at high hardness levels. Under such ii conditions a high degree of resistance to tempering and high toughness characteristics are important.

While we have set out and described a preferred embodiment of our invention, it will be understood that this invention may be otherwise embodied within the scope of the following claims.

We claim:

1. A high speed steel which contains about 0.90 to 1.50% carbon, 1% maximum silicon, 1% maximum manganese, 3% to 6 4% chromium, 3 A to 7%% tungsten, 4% to 8%% molybdenum, 1 A to 3%% vanadium, 9 to 14% cobalt with the balance being substantially all iron and the ratio of vanadium to carbon being about 2 to 1 and the ratio of molybdenum to tungsten being about 6.25 to 5.25.

2. .A high speed steel which contains about 1.00 to 1.30% carbon, .60% maximum silicon, maximum manganese, .30% maximum sulphur, 3% to 5%% chromium, 4% to 6%% tungsten, 5%. to 7 A molybdenum, 11 to 13% cobalt, 1% to 2%% vanadium with the balance being substantially all iron with impurities in conventional amounts, said high speed steel having a ratio of vanadium to carbon of about 2 to l and a ratio of molybdenum to tungsten of about 6.25 to 5.25.

3. A high speed steel which contains about 1.05 to 1.15% carbon, .15 to 35% silicon, .15 to 35% manganese, 4 to 4 /2% chromium, 5 to 5 /z% tungsten, 6 to 6 /2% molybdenum, 2 to 2 /z% vanadium, 11 /2 to 12 /2% cobalt and the balance being substantially all iron with residual impurities in normal amounts, said high speed steel having a ratio of vanadium to carbon of about 2 to 1 and a ratio of cobalt to equivalent tungsten of about .5 to .75. i

4. A high speed steel for cutting applications characterized by high hardness, resistance to tempering in high degree, and excellent cutting qualities, said steel consisting in the following percentages by weight: 1.10% carbon, 4.25% chromium, 2.25% vanadium, 6.25% molybdenum, 5.25% tungsten and 12% cobalt, the balance being substantially all iron with residual irnpurities in conventional amounts.

5. A high speed steel composition hardenable by mantensi-tic and precipitation hardening means resulting in high hardnesses of Rockwell C 68 and above, said steel consisting in the following percentages by weight: 1.05 to 1.15% carbon, 0.15 to 0.35% silicon, 0.15 to 0.35% manganese, 4.0 to 4.5% chromium, 5.0 to 5.5% tungsten, 6.0 to 6.5% molybdenum, 2.0 to 2.5% vanadium, 11.5 to 12.5% cobalt and the balance being substantially all iron, said high speed steel having a ratio of cobalt to equivalent tungsten of about .5 to .75.

References Cited in the file of this patent FOREIGN PATENTS 312,296 Switzerland Mar. 15, 1956 

1. A HIGH SPEED STEEL WHICH CONTAINS ABOUT 0.90 TO 1.50% CARBON, 1% MAXIMUM SILICON, 1% MAXIMUM MANGANESE, 3 1/4 TO 6 1/4% CHROMIUM, 3 1/4 TO 7 1/4% TUNGSTEN, 4 1/4 TO 8 1/4% MOLYBDENUM, 1 1/4 TO 3 1/4% VANADIUM, 9 TO 14% COBALT WITH THE BALANCE BEING SUBSTANTIALLY ALL IRON AND THE RATIO OF VANADIUM TO CARBON BEING ABOUT 2 TO 1 AND THE RATIO OF MOLYBDENUM TO TUNGSTEN BEING ABOUT 6.25 TO 5.25. 